This disclosure relates to seismic surveys and seismic exploration for oil and gas. In particular, but not by way of limitation, this disclosure relates to simultaneous source activation in marine seismic surveying, in which multiple marine seismic vibrators and/or certain signal phase shifts are utilized.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey may involve deploying seismic source(s) and seismic sensors at predetermined locations. The seismic sources generate seismic waves, which propagate into geological formations, creating pressure changes and vibrations along the way. Changes in elastic properties of the geological formation scatter the seismic waves, changing the direction of propagation and other properties of the seismic waves. In a seismic survey, part of the energy emitted by the seismic sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones); other seismic sensors are sensitive to particle motion (e.g., geophones). Seismic surveys may deploy one type of sensor or a combination of both types of sensors. In response to the detected seismic events, the seismic sensors generate seismic data, generally, in the form of electrical signals. Analysis of the seismic data may indicate the presence or absence of probable locations of hydrocarbon deposits.
Some seismic surveys are known as “marine” surveys because the survey is conducted in a marine environment. However, “marine” surveys may not only be conducted in saltwater environments; they also may be conducted in fresh water and brackish water environments. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel.
Other seismic surveys are known as “land” surveys because the surveys are conducted on land environments. Land surveys may use dynamite, seismic vibrators and/or the like as sources. In land surveys seismic sensors/arrays of seismic sensor-containing cables are laid on the ground to receive seismic signals/waves generated by the seismic sources. The seismic signals may be converted, digitized, stored, or transmitted by sensors to data storage and/or processing facilities nearby, e.g. a recording truck. Land surveys may use wireless receivers to avoid the limitations of cables. Seismic surveys may be conducted in areas between land and sea, which are referred to as “transition zones”. Other types of seismic surveys that incorporate both hydrophones and geophones may be conducted on the seabed.
In a conventional towed marine survey using impulsive sources (e.g. an airgun) as a seismic source, a delay is introduced between the firing of one seismic source and the firing of the next seismic source. The delay is selected so as to be of sufficient duration to permit the energy created by the firing of one seismic source to decay to an acceptable level before the energy that is associated with the firing of next seismic source arrives at the seismic sensors. The use of such delays, however, imposes constraints on the rate at which the seismic data may be acquired. For a towed marine survey, these delays require a minimum inline shot interval because the minimum speed of the survey vessel is limited.
To overcome those limitations, simultaneous source activation methods were recently developed. In the simultaneous source activation methods, the delays between source activations/firings are greatly reduced or practically nonexistent. The use of simultaneous sources may increase the data acquisition speed in the seismic survey, lower the overall cost, and provide many other benefits. In simultaneous source activation methods, one seismic source is selected during processing, and seismograms relating to other seismic sources are cancelled by data processing methods to produce a seismogram related to the selected seismic source.
In simultaneous source activation methods, the resulting seismic data (seismograms) from simultaneous sources produce overlapping responses due to the use in the seismic survey of different seismic sources that are fired without the delay described above. In order to separate the seismograms from two or more sources activated at proximal times, there needs to be some feature that distinguishes the seismograms. This feature can come from differences in the impulsive seismogram or from differences in the source output signature or both.
If the seismograms differ significantly, they may be separable using knowledge that some parts of the signal space are occupied only by the part of the seismogram that originates from one of the simultaneous sources. For example, as disclosed in U.S. Pat. No. 5,924,049, to provide for such a significant difference, the sources may emit similar pulses, but are placed at opposite ends of a seismic streamer so that they are separated by a large physical distance. In such a configuration, the two summed simultaneous seismograms occupy predominantly different parts of the Fk spectrum and can be separated by Fk dip filtering. In this configuration, the source signatures of the two simultaneous sources are not well differentiated, but the seismograms from the two are.
Another method is shot-time “dithering,” where relatively small delays (random delays, for example) are introduced between the firings of seismic sources (i.e., the method involves the use of source dithering). The resulting seismic traces are collected into a domain that includes many firings of each seismic source. The traces are aligned such that time zero corresponds to the firing time for a specific source so that the signal(s) acquired due to the specific seismic source appears coherent while the signal(s) acquired due to the other seismic sources appear incoherent. As such, in the dithering methods, the acquired signals may be separated (related to a seismic source) based on coherency. In a typical seismic marine survey, a shot record may last ten (10) seconds. The average time delay or “dithering” time in a dithering-multi-seismic source marine survey may be in the range of one hundred (100) milliseconds or up to many seconds.
In the context of this disclosure, ‘simultaneous’ means sufficiently close in time that the reflected seismic signals generated from two shots (or two sources) overlap in time. In some cases, the seismic sources are close to each other and fired at almost the same time. A shot refers to the activation of a seismic source, which source may be a seismic vibrator, an airgun, a watergun and/or the like.